A vertical temperature stratification develops after a certain time in cryogenic, i.e., very-low-temperature liquids which are at rest, with the topmost layer having the highest temperature. If no foreign gases are contained in the gas space located above the liquid, the pressure in that space and consequently in the entire tank space corresponds to the temperature of the topmost liquid layer, corresponding to the vapor pressure curve applicable to the liquid in question. This means that a pressure that is higher than the pressure corresponding to the mean liquid temperature occurs in the tank. This effect is manifested particularly clearly in stationary tanks, but it also occurs in mobile tanks, which are installed in, e.g., road vehicles or aircraft when the vehicles have been parked for a relatively long time or are in a relatively long, relatively smooth phase of movement without major accelerations. This effect is abolished in a more or less uncontrolled manner during rough travel/rough flight, and a sudden drop in pressure may occur. High tank pressures are disadvantageous for a material-saving tank design of favorable weight, which is used especially in aircraft, because the higher, pressure-related loads also require greater wall thicknesses. It would be possible to eliminate this problem by allowing evaporating or evaporated liquid to escape into the atmosphere by means of a pressure relief valve. However, this would effectively lead to an unintended loss of liquid as well as to environmental pollution.
Another possibility of reducing the pressure would be a continuous or periodic mixing of the amount of liquid with stirrers. Pumping over the contents of the tank could be considered as well. However, such solutions require an expensive design, a number of additional components (motors, stirring blades, pumps, pipelines, valves, etc.), and they also considerably increase the weight of the tank.
The above-mentioned pressure variations during relatively intense movements of the tank are especially disadvantageous as they cause alternating mechanical loads and lead to variations in throughput, i.e., to a nonuniform supply of downstream users.